Assign Pins to Shortest Columns

Pinterest's home feed lays pins out in k vertical columns. Pins arrive one at a time in feed order; each pin i has height heights[i]. Each arriving pin is placed at the bottom of the column whose current total height is smallest; ties go to the leftmost such column.

Return the column index assigned to each pin.

Example:

heights = [5, 3, 4, 2], k = 2
pin0 -> col0 (both 0, leftmost)     cols: [5, 0]
pin1 -> col1 (0 < 5)                cols: [5, 3]
pin2 -> col1 (3 < 5)                cols: [5, 7]
pin3 -> col0 (5 < 7)                cols: [7, 7]
answer: [0, 1, 1, 0]